[Boost-users] not understanding shared_ptr

From: Patrick Loney (Patrick.Loney_at_[hidden])
Date: 2008-03-27 12:37:34

Hi,

I've got some objects which are stored in a DB that are being used in a
multi-threaded application. I'm trying to avoid loading the same object
from the db and would like the added bonus of changes made by one thread
being instantly available to all others (with suitable mutex protection
provided during a change but they are predominantly read-only) 

I've decided that shared_ptrs fit the bill and the following code will
do the job but, never having used them before I was hoping some one
could have a look to make sure I'm not mis-understanding the reason for
shared_ptrs or missing a simpler method to achieve the same results.

Thanks in advance,
Patrick

class dummy
{
public:
        dummy( std::string& id ) : m_id( id ){/*load the object from
db*/}
        virtual ~dummy(){}
        const std::string& objectid(){ return m_id; }
private:
        std::string m_id;
};

typedef boost::shared_ptr<dummy> ListObjectSharedPtr;

dummy& getOrLoadObject( std::string &id )//should I be returning a ref?
{
        static std::list<ListObjectSharedPtr> objectlist;

        for( std::list<ListObjectSharedPtr>::iterator it =
objectlist.begin();
it != objectlist.end(); ++it )
        {
                if( it->get()->objectid() == id )
                        return *it->get();
                if( it->use_count() == 0 )//should this be == 1 as the
object in 							//the
list is always present?
                {
                        objectlist.erase( it );
                        --it;
                }
        }
        ListObjectSharedPtr lo( new dummy( id ) );
        objectlist.push_back( lo );
        return *objectlist.back().get();
}

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